Changes to report natural gas leaks in cubic feet:
    Table NrcUnits: Change standard units for CUBIC FT to CUBIC FT
    from GALLONS.
    Table NrcAnalysis: add comumn 'reported_spill_unit' to store spill volume
    units.
    items.py: add reported_spill_unit field.
    NrcAnalyzer.py fill in reported_spill_unit
    NrcFeedGenerator: report spill volume using stored units, don't
    assume gallons.